The present invention relates to a multi channel temperature controller which performs temperature control by selectively switching over input signals from a plurality of temperature sensors such as thermocouples in a sequential manner, and more particularly relates to such a multi channel temperature controller, particularly improved with regard to its accuracy and its freedom during operation from disturbing influences such as electrical noise.
In the prior art, there have been proposed various types of multi channel temperature controller. Particularly, for selectively switching over a plurality of input signals which are supplied to such a multi channel temperature controller, the switching over action of the contacts of reed relays or the like is conventionally utilized. Such a typical prior art multi channel temperature controller is shown in FIG. 3 of the accompanying drawings in schematic block diagrammatical view. In this figure, the reference symbols S11 through Sn1 denote temperature sensors, and A2 is an amplifier, while Sw11 through Swn1 and Sw12 through Swn2 are reed relays which connect the temperature sensors S11 through Sn1 to the amplifier A2.
Since the signal common amplifier A2 is shared by the plurality of temperature sensors S11 through Sn1, the reed relays Sw11 through Swn1 and Sw12 through Swn2 are necessarily required to be switched over in a sequential manner. In other words, the output signals from the plurality of temperature sensors S11 through Sn1, indicative of the temperatures in the vicinities of said sensors S11 through Sn1, are sequentially supplied to the amplifier A2 via the reed relays Sw11 through Swn1 and Sw12 through Swn2, in order to measure the temperatures in a plurality of locations at which said temperature sensors S11 through Sn1 are disposed. These temperature sensors S11 through Sn1 are provided with compensation circuits at their respective wiring connections.
Now, in order to assure accurate measurements of temperature, since it is typical to use as such temperature sensors S11 through Sn1 such devices as thermo couples which generate relatively weak electro motive force as their output signals, it is necessary to eliminate all extraneous sources of electro motive force in the circuits for the output signals for said temperature sensors S11 through Sn1, i.e. to eliminate all thermal or other electro motive force in said circuits generated outside the locations at which the temperature measurements are conducted.
However, the metal which is utilized for making the contacts of the reed relays Sw11 through Swn1 and Sw12 through Swn2 is generally a metal which is different from the metal, typically copper, which is used for connecting said reed relays Sw11 through Swn1 and Sw12 through Swn2 to the printed circuit board on which typically the whole construction is mounted. This aspect of the problem is particularly illustrated in cross sectional view in FIG. 4 of the accompanying drawings.
In this figure: the reference symbol Sw denotes one of the reed relays Sw11 through Swn1 or Sw12 through Swn2; P is the printed circuit board; M1, M1 are the metal contact members of the reed relay Sw; M2, M2 are copper strips formed on said printed circuit board for conducting the signals to and from this reed relay Sw; and H1 and H2 are portions of solder which are used for joining said metal contact members M1, M1 to said copper strips M2, M2. Since the metal contact members M1, M2 and the copper strips M2, M2 are typically formed from two different metals (i.e. typically the metal contact members M1, M2 are not formed from copper), some thermal electro motive force is inevitably generated in the solder material portions H1, H1. In other words, when electric current is being conducted through this reed relay Sw, some thermal electro motive force is also inevitably generated in said reed relay Sw, and this thermal electro motive force, although external to the temperature sensors (the thermo couples S11 through Sn1 of FIG. 3), i.e. although not being located at the positions at which it is intended to perform temperature measurements, is inevitably inputted to the amplifier A2 as an interference signal. In other words, the voltage that is inputted to said amplifier A2 differs from the voltage that is generated at the thermo couples S11 through Sn1, by this interference electro motive force.
Further, since this is a multi channel temperature controller, since a plurality of the reed relays Sw11 through Swn1 and Sw12 through Swn2 are utilized, and since it is in practice extremely difficult to maintain the temperatures of the surroundings of these reed relays Sw11 through Swn1 and Sw12 through Swn2 substantially identical to one another, the interference voltage produced as described above by each individual one of the reed relays Sw11 through Swn1 and Sw12 through Swn2 is inevitably different.
Moreover, the contacting performance of such a reed relay which includes contacts is not always satisfactory; in fact, such reed relays are very prone to bad contacts.
Accordingly, in the prior art, for the above identified reasons, the problem has occurred that it is not practicable to perform accurate temperature measurements.
Further, the use of such reed relays including contact members is subject to the problems that such reed relays have limited service lives and are not extremely reliable. Certainly such reed relays are not suitable for high speed switching over such as occurs during scanning. Additionally, the need for a relatively large number of reed relays inevitably increases the cost of a multi channel type temperature controller, as well as increasing the size thereof.
Another problem which exists with prior art multi channel type temperature controllers is that, since typically such a multi channel type temperature controller is adapted to serve a large number of different locations for temperature measurement, the length of the wiring between these locations for temperature measurement and the temperature controller which receives signals representative of the measured temperatures tends to be relatively great. And, as the length of such wiring increases, the stray capacitance of such wiring concomitantly inevitably increases.
FIG. 5 of the accompanying drawings shows the influence of ambient noise on the wiring when the length of such wiring is relatively large. In this figure, the reference numeral 1 denotes a temperature sensor, which may be a thermo couple or the like, and which is placed at a location at which the temperature is desired to be measured. And the reference numeral 2 denotes a temperature controller for receiving the signal from the temperature sensor 1, while the reference numeral 3 denotes wiring which connects the temperature sensor 1 to the temperature controller 2.
The symbolic AC voltage Vc1 represents the noise source voltage which is generated in the vicinity of the temperature sensors 1, while the other symbolic AC voltage Vc2 represents a noise source voltage which is generated in the intermediate portions of the wiring 3. And the symbolic capacitance CF1 represents a stray capacitance which is present in the vicinity of the temperature sensor 1, while the other symbolic capacitance CF2 represents a stray capacitance which is present in the intermediate portions of the wiring 3.
As the length of the wiring 3 is increased, capacitive couplings are produced, and the AC components of the resulting common mode voltage noises affect the measurements of temperature which are produced, in the form of errors. And, as the locations for the temperature measurements are changed, so the noise source voltages Vc1 and Vc2 and the stray capacitances CF1 and CF2 are changed. In other words, the common mode voltage noises in the wiring vary depending upon the locations for temperature measurement, since naturally along with such change of the locations for temperature measurement the lengths and the dispositions of the various wiring segments also change.
It would therefore be desirable to eliminate such common mode voltage noise which has an AC component, since it causes errors in temperature measurements as described above. An example of a prior art technique for performing such a process is described in Japanese Patent Publication Serial No. 53-16695 (1978). The outline of this technique will now be described with reference to the schematic diagram given in FIG. 6 of the accompanying drawings.
In FIG. 6, S11, S12, . . . are temperature sensors, and IN11, IN12, . . . are input circuits which use isolation transformers or isolation amplifiers, while MPX1 is an input selection circuit such as an analog type electronic multiplexer which converts a plurality of input signals into a single output signal. SI1 through SI3 are selection signals, and Sout is the output signal.
The values of the common mode voltage noises produced at the input points of the input circuits IN11, IN12, . . . vary depending upon the locations at which the temperature sensors S11, S12, . . . are disposed, and upon the distances between said temperature sensors S11, S12, . . . and the input circuits IN11, IN12, . . . . Therefore, after the common mode voltage noises are eliminated by using the isolation transformers or isolation amplifiers incorporated in said input circuits IN11, IN12, . . . , the signals are supplied to the input selection circuit MPX1, and leak arising from the common mode voltage noises is avoided.
However, this construction entails the need for the input circuits IN11, IN12, . . . to be of isolation types utilizing isolation transformers or isolation amplifiers, and one of these input circuits IN11, IN12, . . . is required to be provided for each of the temperature sensors S11, S12, . . . . Thereby, not only are the size and weight of the multi channel temperature controller increased, but also its manufacturing cost is increased due to the provision of this complicated circuit structure which includes a multitude of constituent parts.